Non-random distribution of deletion endpoints in the gal operon of E. coli

IS4 is still found at its chromosomal site after transposition to galT

IS4-DNA has been hybridized to separated DNA fragments of E. coli K12 strain M28 and to three mutants caused by transposition of IS4 to galT. The parental strain shows one band hybridizing to IS4 representing one copy of IS4 in the chromosome. The mutants have this copy retained and show in addition a second band corresponding to the IS4 copy in galT. The experiments support the hypothesis that transposition of IS4 is accompanied by replication of the element.

Deletion mapping of the niiA niaD gene region of Aspergillus nidulans

The genetic fine-structure of theniiAniaD gene region ofAspergillus nidulanshas been studied using deletion mapping. Deletions were identified asniiAniaD double mutants and comprised 1% of spontaneous chlorate-resistant mutants. All such double mutants were shown to involve deletions and their frequency was not increased by mutagenic treatment with eitherN-methyl-N′-nitro-N-nitrosoguanidine or with ultraviolet light. Deletion maps of theniaD andniiA genes have been constructed. A further class of mutation was also mapped using the deletions. Thesecrnmutations, which affect a gene whose function is as yet unknown, map on the centromere proximal side ofniiA. This analysis of a eukaryote gene cluster will provide a framework upon which to base studies of the regulation of the nitrate assimilation pathway.

Non-random distribution of transduction termini in transductants from the integrated R plasmid, R100-1

Tra+ and tra- derivatives of drug resistance plasmid, R100-1, were isolated by phage P1 from an Hfr donor with integrated R100-1 and then analyzed by complementation tests with tra- point mutants of Flac. Tra+ derivatives of R100-1 carrying tetracycline resistance alone and those carrying all six drug-resistrance genes could support transfer of tra- point mutants of Flac except Flac traJ, whereas all of tra- derivatives of R100-1 failed to complement any one of tra- point mutants of Flac. This suggests that these tra- derivatives of R100-1 carrying tetracycline resistance gene are deleted for all the transfer genes impaired in the Flac point mutants tested. We assume a "hot point", probably a specific base sequence similar to an IS element, at the left of the tetracycline gene (Fig. 1) becomes a transduction terminus in transduction of the integrated R100-1 by phage P1. Complementation analysis of tra- derivatives carrying five resistance genes except the tetracycline gene led us to a supposition that a gene(s), probably analogous to traJ of the F plasmid, is located on R100-1 near the tetracycline gene which plays an important regulatory role for self-transfer as well as for the complementation of tra- Flac mutants.

Transcription of insertion elements IS1 and IS2 in vitro

Insertion elements IS1 and IS2 integrated within the gal operator-promoter region, an IS1 element in gene galT and insertions IS1 and IS2 integrated in the xycIIOP region of phage lambda were transcribed in vitro with E. coli RNA-polymerase. The insertion elements are transcribed exclusively by polymerase molecules started at the gal promoter and the lambdaPR promoter respectively. No promoter exists on IS1 or IS2 which can be recognized by RNA-polymerase in the pure in vitro transcription system used. Both insertions apparently are transcribed with a lower elongation rate than gal operon DNA or lambdaDNA. RNAs transcribed from the termini of IS1 and IS2 respectively were analysed by hybridization experiments. They are different in sequence.

Suppression of polarity of insertion mutations within the gal operon of E. coli

Phenotypic revertants of galOP::IS1 and galOP::IS2 mutations have been isolated after mutagenesis with nitrosoguanidine, they are probably caused by mutations in gene suA. The polarity suppressor mutations described in this study and a known mutation in gene suA isolated by D. Morse (Morse and Guertin, 1972) suppress polarity caused by IS1 more effectively than that caused by IS2 or IS4. Furthermore, suppressibility is influenced by the site and orientation of IS integration. The synthesis of the three enzymes in galOP::IS suA double mutants is constitutive and the ratio of the three enzymes is altered in comparison to the wild type. The reasons for constitutive synthesis of the galactose enzymes and for the altered ratio of enzyme synthesis are discussed.

Gal mRNA initiated within IS2

A strain of E. coli carrying IS2 in the control region of the gal operon has been found to revert to a constitutive phenotype. These revertants can be divided into two classes, which differ in their rate of enzyme synthesis and gal transcription. One revertant synthesizes the gal messenger at a low level (low level revertants), the other at a higher level that exceeds that of the induced wild type two- to three-fold (high level revertants). In both cases it has been shown that the gal messenger is covalently bound to IS2-RNA, which is transcribed from IS2 in the original orientation. The evidence suggests, that the section of the IS2-element, from which this RNA is transcribed in the case of the high level revertants is smaller than 50 nucleotides long: i.e. the resolution of the hybridization method used for the detection of sequence homologies exceeds that of the electron microscopical heteroduplex technique.

Isolation and characterization of phi80dgal transducing phages that carry gal operator-promoter insertion mutations

phi80dgal transducing bacteriophages have been isolated by the F-fusion technique of Press et al. (1971) and gal-operator-promoter insertion mutations have been introduced by homogenate formation. Five different phi80dgal isolates have been studied in more detail. One of the phi80 phages transduces the gal operon and gene aroG as well as at least part of the trp-operon; the gal operon of another phi80dgal transducing phage is inverted with respect to the phi80dgal sequences. Heteroduplex DNA mapping indicates that one of the phi80dgal isolates in addition to the gal operon and a portion of the adjacent chromosomal region carries an IS2-element which is derived from the F'gal episome. The isolated phi80dgal phages may be utilized for preparing pure gal mRNA and insertion-RNA as well as pure gal operon DNA.

Intragenic mutational spectra and hot spots

In this review we outline the various factors which may contribute to the non-randomness of intragenic mutational spectra and the occurrence of hot spots. These factors include sample size limitation, particularly for sites of low mutability, and possible regions of low recombination potential. In addition, the nature of the gene product places great restraint on the detectability of either frameshift and premature chain-terminating mutations on one hand, or of the majority of missense mutations on the other. The nature of the Genetic Code itself also limits the mutational spectrum in so far as specific base pair substitutions lead only to a limited number of detectable amino acid replacements. Mutational hot spots may be a special example of the influence of neighbouring base pairs in the mutability of any given base pair. This is apparently true for frameshift mutations which tend to occur in runs of repeated base pairs or base pair doublets. Neighbouring base effects could operate not only at the level of initial reactivity with a mutagen, but also subsequently at the levels of DNA repair, recombination or replication. In some cases rare or modified bases may be responsible for neighbour effects. We suggest specific experimental approaches which seem likely to aid in the elucidation of these problems.

Isolation and characterization of lambda transducing bacteriophages for the su1+ (supD minus) amber suppressor of Escherichia coli

Specialized lambda transducing phages for the sul+ (supD-) amber suppressor in Escherichia coli K-12 have been isolated, using a secondary site lambda-cI857 lysogen in which we have shown the prophage to be closely linked to sul+.sul+ transducing particles were detected frequently, at 10-5 per plaque-forming unit, in lysates prepared from the secondary-site lysogens. High-frequency transducing lysates were obtained from several independently isolated sul+ transductants and were analyzed by CsCl equilibrium density gradient centrifugation. The transducing phages are defective; marker rescue analysis indicates that the lambda-N gene is not present. In lambda-cI857DELTANdSul+, a bio-type transducing phage, the genes specifying recombination and excision functions have been replaced by bacterial deoxyribonucleic acid.

IS1 is involved in deletion formation in the gal region of E. coli K12

The DNA sequence IS1, which is 800 pairs long, has been shown to integrate into various bacterial and phage operons. The presence of this DNA sequence in the gal operon of E. coli K12 leads to an 30-2000 fold increase in deletion formation in the gal region as compared to wildtype. This high frequency of deletion formation is specific for IS1 and is independent of the cellular recA function. While the frequency of reversion of gal::IS1 mutations, which also is independent of recA, is not affected by the growth temperature of the cells, the formation of deletions in the gal::IS1 system is strongly dependent on the temperature of growth. Mapping experiments showed that one endpoint of the deletions in most cases is at the site of the IS1 mutation and the second endpoint seems to be at various but preferred sites. The formation of the different classes of delections observed is affected differently by the growth temperature of the cells. A model to account for these results is presented.